Fix overflow when logging MaxInt32 to a sparse histogram.

base/metrics/histogram_samples.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_METRICS_HISTOGRAM_SAMPLES_H_
 #define BASE_METRICS_HISTOGRAM_SAMPLES_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 180 matching lines @@
 
 class BASE_EXPORT SampleCountIterator {
  public:
   virtual ~SampleCountIterator();
 
   virtual bool Done() const = 0;
   virtual void Next() = 0;
 
   // Get the sample and count at current position.
   // |min| |max| and |count| can be NULL if the value is not of interest.
+  // Note: |max| is int64_t because histograms support logged values in the
+  // full int32_t range and bucket max is exclusive, so it needs to support
+  // values up to MAXINT32+1.

[Ilya Sherman, 2017/05/02 21:37:49]

Could you add a compile-time assertion somewhere that the max-value for the
recorded-to bucket is less than the max-possible value for the |max| param? 
We've been talking about possibly extending histograms to report 64-bit values,
and I'm a bit worried that we'd accidentally regress this were anyone to
undertake implementing that transition.

[Alexei Svitkine (slow), 2017/05/03 15:21:53]

Done.

   // Requires: !Done();
   virtual void Get(HistogramBase::Sample* min,
-                   HistogramBase::Sample* max,
+                   int64_t* max,
                    HistogramBase::Count* count) const = 0;
 
   // Get the index of current histogram bucket.
   // For histograms that don't use predefined buckets, it returns false.
   // Requires: !Done();
   virtual bool GetBucketIndex(size_t* index) const;
 };
 
 class BASE_EXPORT SingleSampleIterator : public SampleCountIterator {
  public:
   SingleSampleIterator(HistogramBase::Sample min,
-                       HistogramBase::Sample max,
+                       int64_t max,
                        HistogramBase::Count count);
   SingleSampleIterator(HistogramBase::Sample min,
-                       HistogramBase::Sample max,
+                       int64_t max,
                        HistogramBase::Count count,
                        size_t bucket_index);
   ~SingleSampleIterator() override;
 
   // SampleCountIterator:
   bool Done() const override;
   void Next() override;
   void Get(HistogramBase::Sample* min,
-           HistogramBase::Sample* max,
+           int64_t* max,
            HistogramBase::Count* count) const override;
 
   // SampleVector uses predefined buckets so iterator can return bucket index.
   bool GetBucketIndex(size_t* index) const override;
 
  private:
   // Information about the single value to return.
   const HistogramBase::Sample min_;
-  const HistogramBase::Sample max_;
+  const int64_t max_;
   const size_t bucket_index_;
   HistogramBase::Count count_;
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_SAMPLES_H_